大尺寸AMOLED显示的技术挑战

综述了目前大尺寸AMOLED显示的技术挑战,尤其是背板技术。然后论述了如何采用氧化物TFT新技术与LTPS和a-Si TFT的优势相结合制造大尺寸背板的最佳方案。通过对比传统准分子激光退火（ELA）LTPS和非晶铟-镓-氧化锌（a-IGZO）TFT的器件特性,特别揭示了氧化物TFT的挑战性技术。最后,展示了由a-IGZO TFT背板制造的12.1in WXGAAMOLED显示器原型机。     

Amorphous Silicon Thin-Film Transistor Backplanes Deposited at 200 °C on Clear Plastic for Lamination to Electrophoretic Displays

The transition of thin-film transistor (TFT) backplanes from rigid plate glass to flexible substrates requires the development of a generic TFT backplane technology on a clear plastic substrate. To be sufficiently stable under bias stress, amorphous-silicon (a-Si:H) TFTs must be deposited at elevated temperatures, therefore the substrate must withstand high temperatures. We fabricated a-Si:H TFT backplanes on a clear plastic substrate at 200degC. The measured stability of the TFTs under gate bias stress was superior to TFTs fabricated at 150degC. The substrate was dimensionally stable within the measurement resolution of 1, allowing for well-aligned 8 times 8 and 32 times 32 arrays of pixels. The operation of the backplane is demonstrated with an electrophoretic display. This result is a step toward the drop-in replacement of glass substrates by plastic foil.     

Stability of amorphous-silicon TFTs deposited on clear plastic substrates at 250/spl deg/C to 280/spl deg/ C

Amorphous-silicon (a-Si) thin-film transistors (TFTs) were fabricated on a free-standing new clear plastic substrate with high glass transition temperature (T/sub g/) of >315/spl deg/ C and low coefficient of thermal expansion of <10 ppm/ /spl deg/ C. Maximum process temperatures on the substrates were 250/spl deg/C and 280/spl deg/C, close to the temperatures used in industrial a-Si TFT production on glass substrates. The first TFTs made at 280/spl deg/C have dc characteristics comparable to TFTs made on glass. The stability of the 250/spl deg/C TFTs on clear plastic is approaching that of TFTs made on glass at 300/spl deg/C-350/spl deg/C. TFT characteristics and stability depend only on process temperature and not on substrate type.     

Single-Technology-Based Statistical Calibration for High-Performance Active-Matrix Organic LED Displays

Active-matrix organic light-emitting-diode (AMOLED) displays based on amorphous hydrogenated silicon (a-Si:H) thin-film transistors (TFTs) are the state of the art in display technology, owing to the feasilibility of low-cost fabrication and accessability to well-established TFT-LCD fabrication. While the a-Si:H TFT offers excellent matching of device properties over large areas, it suffers from a gate-bias-dependent threshold voltage shift in time, leading to grayscale inaccuracies. In order to counter this problem, many compensation circuits have been designed. The purpose of the compensation circuit is to estimate the threshold voltage shift in driver TFTs and apply a correction so as to maintain a constant brightness. However, all of the compensation circuits designed to date suffer from low spatial and temporal resolution and reliability issues or high cost due to the use of custom-made CMOS technology. In this paper, we focus on building AMOLED display systems solely based on a-Si:H TFT technology along with the use of off-the-shelf CMOS components to lower costs. Furthermore, we achieve high spatial and temporal resolution and high yield with the use of a two-TFT voltage programmed pixel circuit along with a statistical based external calibration circuit.     

A driving scheme for active-matrix organic light-emitting diode displays based on feedback

A scheme of driving active matrix organic light emitting diode (AMOLED) displays with hydrogenated amorphous silicon (a-Si) thin-film transistors (TFTs) is presented. By sending a feedback voltage from each pixel to a column driver during the programming cycle, the driving scheme can compensate for the instability of the TFTs, in particular, the shift in the threshold voltage. Measurement results show no change in the OLED current in the presence of a 1.3-V shift in the threshold voltage. Based on circuit analysis, a simple lead compensator and an accelerating pulse were employed to achieve fast pixel programming for a wide range of OLED currents. Simulation results show a programming time of less than 70 /spl mu/s for OLED currents as low as 50 nA.     

Device and circuit level optimization for high performance a-Si:H TFT-based AMOLED displays

Active matrix organic light-emitting diode (AMOLED) displays with amorphous hydrogenated silicon (a-Si:H) thin-film transistor (TFT) backplanes are becoming the state of art in display technology. Though a-Si:H TFTs suffer from an intrinsic device instability, which inturn leads to an instability in pixel brightness, there have been many pixel driving methods that have been introduced to counter this. However, there are issues with these circuits which limit their applicability in terms of speed and resolution. This paper highlights these issues and provides detailed design considerations for the choice of pixel driver circuits in general. In particular, we discuss the circuit and device level optimization of the pixel driver circuit in a-Si:H TFT AMOLED, displays for high gray scale accuracy, subject to constraints of power consumption, and temporal and spatial resolution.     

Stability of Amorphous-Silicon and Nanocrystalline Silicon Thin-Film Transistors Under DC and AC Stress

Bottom-gated n-channel thin-film transistors (TFTs) were fabricated using hydrogenated amorphous-silicon (a-Si:H)/ nanocrystalline silicon (nc-Si:H) bilayers as channel materials, which are deposited by plasma-enhanced chemical vapor deposition at low temperatures. The stability of these devices is investigated under static and dynamic bias stress conditions. For comparison, the stability of a-Si:H and nc-Si:H single-layer TFTs is investigated under similar bias stress conditions. The overall results demonstrate that the a-Si:H/nc-Si:H bilayer TFTs are superior compared with their counterparts of a-Si:H and nc-Si:H TFTs regarding device performance and stability.     

Integration and characterization of amorphous silicon thin-filmtransistor and Mo-tips for active-matrix cathodes

We have designed and monolithically integrated amorphous silicon thin-film transistor (a-Si TFT) with Mo-tip field emitter arrays (FEAs) on glass substrate for active-matrix cathodes (AMCs) in field-emission display (FED) application. In our AMCs, a light shield layer of metal was introduced to reduce the photo leakage and back channel currents of a-Si TFT. The light shield was designed to have the role of focusing grid to focus emitted electron beams from the AMC on the corresponding anode pixel by forming it around the Mo-tip FEAs as well as above the a-Si TFT. The thin film depositions in a-Si TFTs were performed at a high temperature of above 360°C to guarantee the postvacuum packaging process of cathode and anode plates in FED. Also, a novel wet etching process was developed for n⁺-doped-a-Si etching with high etch selectivity to intrinsic a-Si and good etch controllability and was used in the fabrication of inverted stagger TFT with a very thin active layer. The developed a-Si TFTs had good enough performance to be used as control devices for AMCs with Mo-tip emitters. The fabricated AMCs exhibited very effective aging process for field emitters     

Integration of organic LEDs and amorphous Si TFTs onto flexible andlightweight metal foil substrates

We report the integration of organic light emitting devices (OLEDs) and amorphous Si (a-Si) thin-film transistors (TFTs) on both glass, and unbreakable and lightweight thin stainless steel foil substrates. The doped-polymer OLEDs were built following fabrication of driver TFTs in a stacked structure. Due to the opacity of the steel substrate, top-emitting OLED structures were developed. It is shown that the a-Si TFTs provide adequate current levels to drive the OLEDs at video brightness (~100 cd/m²). This work demonstrates that lightweight and rugged TFT backplanes with integrated OLEDs are essential elements for robust and highly portable active-matrix emissive flat-panel displays     

Does TFT mobility impact pixel size in AMOLED backplanes?

Recent advances in organic light emitting diode (OLED) device efficiencies are making the amorphous silicon (a-Si) backplane a viable solution for a large range of display sizes. This paper presents the possibilities and design challenges of a-Si active-matrix organic light-emitting-diode (AMOLED) backplanes for applications ranging from small full color cell phone displays to HDTV screens. An analytical model for the minimum pixel-area, and hence the maximum resolution for both bottom and top emitting AMOLED architectures is presented in terms of the a-Si thin-film transistor (TFT) device parameters, the process design-rules, and the pixel circuit parameters. It is established that the lower device mobility of a-Si TFTs is no longer the limiting factor. For instance, in a 20' W/SXGA panel with full color red-green-blue subpixels, the state-of-the-art TFT processes yield a square pixel size of /spl sim/266 /spl mu/m. Further, quantitative analysis of charge-injection/charge-feedthrough error in the pixel, and the maximum allowable leakage current for the TFT is also presented.     

A new a-Si:H TFT pixel circuit compensating the threshold voltage shift of a-Si:H TFT and OLED for active matrix OLED

We propose a new hydrogenated amorphous silicon thin-film transistor (a-Si:H TFT) pixel circuit for an active matrix organic light-emitting diode (AMOLED) employing a voltage programming. The proposed a-Si:H TFT pixel circuit, which consists of five switching TFTs, one driving TFT, and one capacitor, successfully minimizes a decrease of OLED current caused by threshold voltage degradation of a-Si:H TFT and OLED. Our experimental results, based on the bias-temperature stress, exhibit that the output current for OLED is decreased by 7% in the proposed pixel, while it is decreased by 28% in the conventional 2-TFT pixel.     

A Driving Scheme for Active-Matrix Organic Light-Emitting Diode Displays Based on Current Feedback

This paper presents a method of driving active-matrix organic light-emitting diode (AMOLED) displays with amorphous silicon (a-Si) thin-film transistors (TFTs). By using current feedback, the method effectively compensates for the effect of shift in the threshold voltage $(V_{T})$ of a-Si TFTs on the OLED current. A CMOS transresistance amplifier is used as the column driver to cancel the effect of large parasitic capacitance of data lines. An accelerating pulse is used at the start of the programming cycle to improve the settling at low currents. A detailed analysis has been done to investigate the effect of circuit components on the sensitivity of the OLED current to $V_{T}$ shift and the settling behavior of the circuit. Prototypes of pixel circuits and the transresistance amplifier were fabricated in an a-Si TFT process and a 0.8- $mu{hbox{m}}$ 20-V CMOS technology, respectively. Measurements show less than 5% change in the OLED current for 2.5-V shift in $V_{T}$ of TFTs. Settling times smaller than 50 $mu{hbox{s}}$ were achieved for parasitic capacitances of 50–200 pF and programming currents as small as 200 nA.      

Excimer-laser crystallized poly-Si TFT's with transparent gate

A new poly-crystal silicon thin-film transistor (poly-Si TFT) with a transparent bottom-gate electrode has been fabricated by XeF excimer-laser light irradiation from the glass substrate side. Compared with poly-Si TFTs made by XeF or ArF excimer-laser light irradiation to the top Si surface, the new TFT shows a higher electron mobility of about 100 cm²/Vs, independent of the Si film thickness. Therefore, poly-Si driver TFTs and amorphous-silicon (a-Si) TFTs for the matrix can be formed with the same channel-etch type bottom-gate structure simultaneously on the same glass substrate by using the same starting materials. This is expected to open the way for making driver monolithic and active matrix liquid crystal displays     

A Stable Voltage-Programmed Pixel Circuit for a-Si:H AMOLED Displays

Hydrogenated amorphous silicon (a-Si:H) active matrix organic light-emitting diode (AMOLED) displays are attractive given the potentially low manufacturing cost and ultimately low-temperature fabrication enabling using flexible substrates. Although the conventional two thin-film transistor (2-TFT) AMOLED voltage-programmed pixel circuit (VPPC) can provide high resolution and high yield, the 2-TFT VPPC is prone to image retention over time due to shift in the threshold voltage (V_T-shift) of a-Si:H TFTs. This paper presents a new driving scheme that not only preserves the simplicity of the 2-TFT VPPC, but also demonstrates high uniformity. Experimental results indicate that the current drop in the new driving scheme is less than 11% after 15 days of operation whereas it is over 50% for the conventional driving scheme. Moreover, the new driving scheme is less sensitive to temperature variations due to an internal feedback mechanism. After a 70% change in the temperature, the current in the conventional driving scheme increases by as much as 300%. However, the current in the driving scheme presented here is approximately constant     

Stability of nc-Si:H TFTs With Silicon Nitride Gate Dielectric

We report the fabrication and characterization of bottom-gate and top-gate nanocrystalline silicon (nc-Si:H) thin-film transistors (TFTs) with amorphous-silicon nitride (a-SiN_x:H) as the gate dielectric. The devices were fabricated using standard 13.56-MHz plasma-enhanced chemical vapor deposition at 240 degC. Here, the same 80-nm nc-Si:H channel, 300-nm a-SiN_x:H gate dielectric, and 60-nm n⁺ nc-Si:H ohmic contact layers were used in both TFT structures. We analyzed the effects of gate configuration on TFT performance and, in particular, the electrical stability. The stability tests were carried out at a gate bias stress in the range from 20 to 40 V. The nc-Si:H TFTs demonstrated much better threshold-voltage (V_T ) stability compared with the amorphous-silicon (a-Si:H) counterparts, offering great promise for applications in active-matrix organic light-emitting diode (AMOLED) displays     

Amorphous Silicon Display Backplanes on Plastic Substrates

Amorphous silicon (a-Si) thin-film transistor (TFT) backplanes are very promising for active-matrix organic light-emitting diode displays (AMOLEDs) on plastic. The technology benefits from a large manufacturing base, simple fabrication process, and low production cost. The concern lies in the instability of the TFTs threshold voltage (V_T) and its low device mobility. Although V_T-instability can be compensated by means of advanced multi-transistor pixel circuits, the lifetime of the display is still dependent on the TFT process quality and bias conditions. A-Si TFTs with field-effect mobility of 1.1 cm²/Vmiddots and pixel driver circuits have been fabricated on plastic substrates at 150 degC. The circuits are characterized in terms of current drive capability and long-term stability of operation. The results demonstrate sufficient and stable current delivery and the ability of the backplane on plastic to meet AMOLED requirements     

Low-Temperature Materials and Thin Film Transistors for Flexible Electronics

This paper addresses the low-temperature deposition processes and electronic properties of silicon based thin film semiconductors and dielectrics to enable the fabrication of mechanically flexible electronic devices on plastic substrates. Device quality amorphous hydrogenated silicon (a-Si:H), nanocrystalline silicon (nc-Si), and amorphous silicon nitride (a-SiN/sub x/) films and thin film transistors (TFTs) were made using existing industrial plasma deposition equipment at the process temperatures as low as 75/spl deg/C and 120/spl deg/C. The a-Si:H TFTs fabricated at 120/spl deg/C demonstrate performance similar to their high-temperature counterparts, including the field effect mobility (/spl mu//sub FE/) of 0.8 cm/sup 2/V/sup -1/s/sup -1/, the threshold voltage (V/sub T/) of 4.5 V, and the subthreshold slope of 0.5 V/dec, and can be used in active matrix (AM) displays including organic light emitting diode (OLED) displays. The a-Si:H TFTs fabricated at 75/spl deg/C exhibit /spl mu//sub FE/ of 0.6 cm/sup 2/V/sup -1/s/sup -1/, and V/sub T/ of 4 V. It is shown that further improvement in TFT performance can be achieved by using n/sup +/ nc-Si contact layers and plasma treatments of the interface between the gate dielectric and the channel layer. The results demonstrate that with appropriate process optimization, the large area thin film Si technology suits well the fabrication of electronic devices on low-cost plastic substrates.     

External Compensation of Nonuniform Electrical Characteristics of Thin-Film Transistors and Degradation of OLED Devices in AMOLED Displays

The variation of electrical characteristics of polycrystalline-silicon thin-film transistor (TFT) and degradation of organic light-emitting-diode (OLED) device cause nonuniform intensity of luminance and image sticking in active-matrix OLED (AMOLED) displays. An external compensation method that senses and compensates variations of threshold voltage and mobility of TFTs and degradation of OLED device is proposed. The effect of the external compensation method on AMOLED pixel is experimentally verified by measuring the luminance of OLEDs and the electrical characteristics of TFTs in AMOLED pixels.      

基于P-Type多晶硅TFT技术的集成型有源OLED驱动电路

低温多晶硅(LTPS:Low-temperature poly-Si)技术已经成为薄膜晶体管(TFT:thin film transistor)制作中最具吸引力的技术,并应用在AMOLED显示器中.P-type 技术能够简化 TFT 的制作过程.本文提出了一种应用 p-type 多晶硅 TFT的 AMOLED 驱动电路结构,包括栅极驱动器、数据驱动器以及像素阵列.数据驱动器采用分块方法,使得显示屏的输出线数大大减少.作者采用一种改进的 p-type 移位寄存器实现逐行选通的功能,并采用由 4 个 p-type 反相器级联构成的缓冲器来提高电路的驱动能力.为了验证上述电路结构的正确性,作者采用 HSPICE 软件进行仿真分析.结果表明,电路工作正常.利用韩国汉城国立大学及 Neo Poly 公司在多晶硅制作方面的优势,我们已经合作完成了应用上述电路结构的分辨率为96×3×128的有源 OLED 的制作.     

Electric field-enhanced metal-induced lateral crystallization and P-channel poly-Si TFTs fabricated by it

The poly-Si thin film was obtained by electric field-enhanced metal-induced lateral crystallization technique at low temperature. Raman spectra, X-ray diffraction (XRD) and scan electron microscope (SEM) were used to analyze the crystallization state, crystal structure and surface morphology of the poly-Si thin film. Results show that the poly-Si has good crystallinity, and the electric field has the effect of enhancing the crystallization when DC electric voltage is added to the film during annealing. Secondary ion mass spectroscopy (SIMS) shows that the metal Ni improves the crystallization by diffusing into the a-Si thin film, so the crystallization of the lateral diffused region of Ni is the best. The p-channel poly-Si thin film transistors (TFTs) were fabricated by this large-size grain technique. The I_DS−V_DS and the transfer characteristics of the TFTs were measured, from which, the hole mobility of TFTs was 65 cm²/V s, the on and off current ratio was 5×10⁶. It is a promising method to fabricate high-performance poly-Si TFTs at low temperature for applications in AMLCD and AMOLED.